How to add many strings in c++

First of all, you can do the +sn thing just fine. Though it's going to take exponential quadradic(see comments) time assuming you're using std::basic_string<t> strings on C++03.

You can use the std::basic_string<t>::append in concert with std::basic_string<t>::reserve to concatenate your string in O(n) time.

EDIT: For example

string a;
//either
a.append(s1).append(s2).append(s3);
//or
a.append("I'm a string!").append("I am another string!");

s = s1 + s2 + s3 + .. + sn;

will work although it could create a lot of temporaries (a good optimizing compiler should help) because it will effectively be interpreted as:

string tmp1 = s1 + s2;
string tmp2 = tmp1 + s3;
string tmp3 = tmp2 + s4;
...
s = tmpn + sn;

An alternate way that is guaranteed not to create temporaries is:

s = s1;
s += s2;
s += s3;
...
s += sn;

std::ostringstream is build for that, see example here. It's easy:

std::ostringstream out;
out << "a" << "b" << "c" << .... << "z";
std::string str( out.str());

Use a template to add strings, char* and char's to form a string

strlen:-

#include <iostream>
#include <cstring>

// it_pair to wrap a pair of iterators for a for(:) loop
template<typename IT> 
class it_pair
    {
    IT b;
    IT e;
public:
    auto begin() const
        {
        return b;
        }
    auto end() const
        {
        return e;
        }
    };

// string length
template<typename S> auto strlen(const S& s) -> decltype(s.size())
    {
    return s.size();
    }

auto strlen(char c) -> size_t
    {
    return 1u;
    }

auto strlen(const std::initializer_list<char>& il) -> size_t
    {
    return il.size();
    }

template<typename IT>
auto strlen(const it_pair<IT>& p)
    {
    auto len = size_t{};
    for(const auto& s:p)
        len += strlen(s);
    return len;
    }

template<typename S, typename ...SS> auto strlen(S s, SS&... ss) -> size_t
    {
    return strlen(s) + strlen(ss...);
    }

appending strings

// terminate recursion
template<typename TA, typename TB>
void append(TA& a, TB& b)
    {
    a.append(b);
    }

// special case for a character
template<>
void append<std::string, const char>(std::string& a, const char& b)
    {
    a.append(1, b);
    }

// special case for a collection of strings
template<typename TA, typename TB>
void append(TA& a, const it_pair<TB>& p)
    {
    for(const auto& x: p)
        a.append(x);
    }

// recursion append
template<typename TA, typename TB, typename ...TT>
void append(TA& a, TB& b, TT&... tt)
    {
    append(a, b);
    append(a, tt...);
    }

template<typename ...TT>
std::string string_add(const TT& ... tt)
    {
    std::string s;
    s.reserve(strlen(tt...));
    append(s, tt...);
    return s;
    }

template<typename IT>
auto make_it_pair(IT b, IT e)
    {
    return it_pair<IT>{b, e};
    }

template<typename T>
auto make_it_pair(const T& t)
    {
    using namespace std;
    return make_it_pair(cbegin(t), cend(t));
    }

main example

int main()
    {
    const char * s[] = {"vw", "xyz"};
    std::vector<std::string> v{"l", "mn", "opqr"};
    std::string a("a");
    std::string b("bc");
    std::string c("def");
    std::cout << string_add(a, b+c, "ghij", make_it_pair(v), 'k', make_it_pair(s));
    }

If you want to be able to do this

• Efficiently, i.e without incurring quadratic time
• Without overwriting the original variable
• Without creating temporary variables

Then this will do the job

auto s = std::string(s1).append(s2).append(s3).append(sn);  

And if you like things nicely formatted

auto s = std::string(s1).append(s2)
                        .append(s3)
                        .append(sn)